The present invention relates to a method and device for separating ions according to a physicochemical property, such as ion mobility or mass to charge ratio.
A conventional ion mobility spectrometer or separator (“IMS”) employs a static potential gradient to drive ions along a drift tube such that the ions separate according to their ion mobility as they pass along the drift tube. It is well established that increasing the length of the drift tube can lead to improved resolution or resolving power of the device. This is also true for IMS devices that employ a travelling DC potential, rather than a static potential gradient, to drive the ions along the drift tube. Such a device employing a DC travelling potential has the advantage that there is no requirement for a DC field along the whole length of the drift tube. This enables the entrance of the IMS device to be at the same DC potential as the exit of the device.
WO 2007/066114 discloses an IMS device that exploits the above-mentioned benefit by configuring the drift tube as a closed loop configuration, which enables the drift length of the device to be increased by causing ions to perform multiple passes around the closed loop drift region. Whilst this technique enables the effective length of the drift region to be increased significantly, a problem arises when the ions are to be ejected from the device for further analysis. More specifically, at any given position within the device ions of different mobilities will have undergone a different number of cycles around the closed loop drift region. This leads to the resolution of the device being ion mobility dependent, which is undesirable.
WO 2013/093515 discloses a device wherein ions are separated according to their ion mobility as they pass down a DC potential gradient arranged around a closed-loop drift region. The DC potential gradient moves around the drift region with time, along with an exit region, such that ions are ejected from the drift region as they reach the lower end of the DC potential gradient. Although this technique offers advantages, it may be complicated to implement, as it requires the exit region to move around the device. Furthermore, ions of different ion mobilities travel different lengths around the drift region before being ejected and so the resolution of the device is ion mobility dependent.
WO 2013/093513 also discloses a device wherein ions are separated according to their ion mobility as they pass down a DC potential gradient arranged around a closed-loop drift region. The device is configured to ensure that higher mobility ions do not pass around the closed-loop ion guide and remix with the lower mobility ions by restricting the drift length of the experiment. Although this technique offers advantages, it may provide a relatively low resolution as it restricts the number of cycles of the ions around the closed loop drift region.
It is desired to provide an improved method of separating ions according to a physicochemical property.